# Modulated Light Barrier

Posted on Feb 5, 2014

The light barrier described here can be used to guard an entrance. You can use it to signal of someone is walking through the corridor, or to check if the car has been parked far enough in the garage to be able to close the door. The circuit consists of a transmitter, which sends modulated infrared light and a receiver, which recognises this. The circuit used here is almost insensitive to daylight or fluorescent light and

therefore can be used outside. The transmitter (Figure 1) generates about 1000 times per second, for a period of 540 ms, a burst of 36 kHz. IC1 has been set with C1, R1 and R2 to a frequency of about 1000 Hz. The output of IC1 ensures that IC2 will oscillate about 1000 times per second for a period of about 540 ms. IC2 is set to a frequency of 36 kHz with C2, P1, R4 and R5. The output of IC2 drives the IR LED D1 via transistor T1. C3 and R3 prevent the reasonably high current through D1 from generating too much interference on the power supply rail. The receiver (Figure 2) is quite a simple design, because IC3 already does a lot of the work for us. When the IC sees` an IR-signal with a frequency of 36 kHz, the output of IC3 will become 0`. The transmitter circuit alternates between sending an IR-signal of 36 kHz for 540 ms and is quiet for 470 ms. When this signal arrives at IC3, C4 will discharge via D2. Because the non-inverting input of IC4a is set to 2. 5 V, with the aid of R10 and R11, the output of IC4a will be a 1`. In the intervening quiet periods of 470 ms, C4 will partially charge via R8, but this is not of sufficient duration to exceed the voltage of 2. 5 V. Only when the light barrier is interrupted will C4 charge far enough that the output of IC4a will toggle and become a 0`. Because IC4a has an open-collector output, C5 will be immediately discharged and the output of IC4b will become a 1`. With R9 and C5...

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